// 变量提升
console.log(z); // ✅ 输出undefined  
var z = 10;

// 暂时性死区
console.log(x); // ReferenceError: Cannot access 'x' before initialization
let x = 10;

// const
const y = 10;
y = 20 // TypeError: Assignment to constant variable.

console.log('z, x, y', z, x, y)

